NAD C-725BEE No Sound

Hello there,

I recently got  NAD C-725BEE stereo receiver from one of my friend's place. the problem is that no sound is coming from the speakers when the RCA output of my DAC is connected to CD, AUX, MP, Tape etc input of the NAD. But when the same RCA output is connected to MAIN input, the speakers start sounding. I found out the servicing manual from this website and i saw that Main input directly go to the amplifier circuit, CD,AUX,MP Tape etc goes to the pre amplifier circuit. But I have not opened the receiver yet. Is there any checks i can do at home? I have soldering iron and basic meter stuff with me so i was wondering if i could do some checks myself. if you guys had this issue before, please help me with this trouble.